<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>背景透明</title>
	<style>
		.alpha {
			width: 300px;
			height: 300px;
			
		}

		.alpha:hover{
			background-color: rgba(0, 0, 0, .5);
		}
	</style>
</head>
<body>
	<div class="alpha">
		<h1>我是h1标签</h1>
	</div>
	css三大特性，优先级，层叠性，继承
</body>
</html>